I was told when I was young that Black people because of their buid up and heavy bones that they don't swim well... yet the excel in sprint...
Some black swimmers are fighting this idea - I don't believe there has ever been any science to back it up.
Seeing that when swimming your body is under the water most of the time, and it is clear that black people don't just sink, it is very likely it is an urban myth.
Maybe it was even created by white people to deter blacks from trying swimming - you wouldn't try something that might kill you!
Found some limited research .... the extra bone mass in a black man is / could be around 300 grams - that is negligible:
https://swimmingwithoutstress.co.uk/2014/05/16/black-men-cant-swim/
"Matt Bridge goes on to say that in the US marine corps, where non swimmers have to learn to swim, no one, black or white, has ever failed (quite stringent and vigorous) swimming test. This shows that social factors, confidence, motivation, peer group influences, are significant."
Seems like, once again, it is good old fashioned suppression and racism, that is the reason for so few black people in swimming.
Here's another black swimmer that did OK